So at the risk of being redundant, but in the hope of helping with your planning, this is how I think of it.
AC basically lets you store a patient's documents in two places: Imported items and past encounters. You can put your report in either of these.
To place them in Imported items, you create the report with the letter writer. Advantages: you can title it as you choose, and place it in a folder to sort them, so for example, EKG reports are grouped together. Disadvantages: the folder headings are not easily customizable (they must be changed patient by patient), and items can get "buried" if you have many imported items.
To place them in Past encounters, you create them as an addendum. Advantages: they are listed neatly, chronologically and easy to see in the past encounters box. Disadvantage: they are all called "addendum" and are interspersed with notes and messages.
You might find that either works best for you.
In my mind, improvements to the letter writer and ability to customize Imported Items folders would dramatically improve this aspect of the program and would make it more appealing to specialists in particular.